Locking compression members for use with bone anchor assemblies and methods

ABSTRACT

Various exemplary methods and devices are provided for fixing bone anchors to bone. In general, the methods and devices can allow for a bone anchor to be fixed to a bone at a desired angle to a surface of the bone. In an exemplary embodiment, a bone anchor assembly is provided that includes a bone anchor configured to engage bone, a receiver member for seating a head of the bone anchor, and a compression member for securing the bone anchor at fixed angle with respect to the receiver member when the compression member is seated within the receiver member. Corresponding engagement features of the compression member and the receiver member can be engaged in a secured configuration to inhibit or prevent removal of the compression cap from the receiver member and optionally to substantially prevent longitudinal and/or rotational movement of the compression member with respect to the receiver member.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/827,092 filed on Mar. 14, 2013, now issued as U.S. Pat. No.9,259,247, which is hereby incorporated herein by reference in itsentirety.

FIELD

The present invention relates to methods and devices for correcting aspine, and in particular to bone anchor assemblies and methods of usingthe same.

BACKGROUND

Bone anchors may be used in orthopedic surgery to fix bone duringhealing or during a fusion process. In spinal surgery, bone anchors maybe used with spinal fixation elements, such as spinal rods, to stabilizemultiple vertebrae either rigidly, in which no relative motion betweenthe vertebrae is desired, or dynamically, in which limited, controlledmotion between the vertebrae is desired. Fixation elements can help tosupport the spine in a desired alignment, for example by defining ashape towards which a deformed spine is to be corrected. Attaching thevertebrae to the fixation element causes vertebrae which are out ofposition to be drawn towards the fixation element, so that they can thenbe retained in a correct alignment against forces imposed by soft tissuetending to revert the configuration of the spine towards the deformedshape. Correction of the spinal deformation can involve application tothe vertebrae of translational forces, torsional forces, or combinationsthereof to cause vertebrae to translate and/or rotate.

Surgical procedures using bone anchors often require that the boneanchor be secured to the bone at a predetermined angle to a surface ofthe bone. Traditional bone anchors can include a shaft having aspherical head that is polyaxially seated in a receiver member and thatcan be secured at a fixed angle to the receiver member by a compressionmember. To reduce a risk that the compression member is misplaced ordropped into a surgical incision in a patient before the compressionmember is secured proximally of the head, traditional methods canrequire deformation of the receiver member against the compression cap.The deformation process, called “swaging,” compresses the receivermember against the compression cap to substantially prevent relativemovement therebetween. However, the swaging process can weaken thestructure of the bone anchor and can increase the expense and the timerequired for manufacturing of the bone anchor.

Accordingly, there remains a need for improved methods and devices forbone anchor fixation.

SUMMARY

The present invention generally provides methods and devices for fixinga bone anchor to a bone. In one aspect, a bone screw assembly isprovided that can include a screw having a proximal portion and a distalshank portion, a receiver member having a polyaxial seat formed thereinand configured to proximally seat the head portion of the screw, and acompression cap. The compression cap can be disposed within the receivermember and can have a distal end configured to engage the head portionof the screw. Opposed projections of the compression cap can extendradially therefrom and can be configured to mate with complementaryrecesses formed in the receiver member such that the compression cap isretained within the receiver member.

The opposed projections can be configured in any number of ways. Adistance between outer surfaces of the opposed projections can begreater than the inside diameter of the receiver member, although thecompression cap can have an outside diameter that is less than an insidediameter of the receiver member. A width of each projection measuredhorizontally about the circumference of the compression cap can begreater than a thickness of the opposed projection measured along aradial axis of the compression cap. The opposed projections can beconfigured such that movement of the compression cap distally within thereceiver member is effective to deform the opposed projections until theopposed projections snap into engagement with the complementaryrecesses. Rotating the compression cap relative to the receiver membercan also be effective to deform the opposed projections until theopposed projections snap into engagement with the complementaryrecesses, and/or can be effective to move the opposed projections intoengagement with the complementary recesses. The opposed projections andthe complementary recesses can be effective, when mated, to maintain thecompression cap in a substantially fixed longitudinal and/or rotationalposition relative to the receiver member.

A proximal-facing surface of each projection and an inner superiorsurface of each complementary recess can be planar such that, whenmated, the compression cap is prevented from being decoupled from thereceiver member. A distal-facing surface of each projection can beramped, tapered, chamfered, and/or beveled to provide a lead-in surfacegeometry. Similarly, the receiver member can include a shoulder disposedproximal to the complementary recesses, the shoulder being ramped,tapered, chamfered, and/or beveled to provide a lead-in surface for theopposed projections as the compression cap is advanced distally relativeto the receiver member. Where the complementary recesses comprisecut-outs formed in a shelf that extends radially inward from an interiorsidewall of the receiver member, the shelf can include one or morelateral edges that are ramped, tapered, chamfered, and/or beveled toprovide a lead-in surface for the opposed projections.

In another aspect, a method is provided for assembling a bone screw thatcan include passing a shank portion of the bone screw through anaperture formed in a distal end of a receiver member, inserting acompression cap into the receiver member, and engaging opposedprojections extending radially outward from the compression cap withcomplementary recesses formed in the receiver member to retain thecompression cap within the receiver member. The method can furtherinclude deforming the projections over a shoulder portion of thereceiver member disposed proximal to the complementary recesses. Theengaging can be effective to maintain the compression cap in asubstantially fixed longitudinal and/or rotational position relative tothe receiver member. Inserting the compression cap can comprise slidingthe compression cap distally within the receiver member to deform theopposed projections until the opposed rejections snap into engagementwith the complementary recesses. Inserting the compression cap can alsocomprise rotating the compression cap relative to the receiver member tomove the opposed projections into engagement with the complementaryrecesses. Where the complementary recesses can be formed in a shelf thatextends radially inward from an interior sidewall of the receivermember, the engaging can comprise camming the opposed projections overramped, tapered, chamfered, and/or beveled edges of the shelf andsnapping the opposed projections into the complementary recesses.

The present invention further provides devices, systems, and methods asclaimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be more fully understood from the following detaileddescription taken in conjunction with the accompanying drawings, inwhich:

FIG. 1A (PRIOR ART) is a perspective view of a prior art bone anchorassembly;

FIG. 1B (PRIOR ART) is an exploded view of the bone anchor assembly ofFIG. 1A;

FIG. 1C (PRIOR ART) is a top view of the bone anchor assembly of FIG.1A;

FIG. 1D (PRIOR ART) is a cross-sectional view of the bone anchorassembly of FIG. 1A;

FIG. 2 is a perspective, partially exploded view of a bone anchorassembly including a bone anchor, a receiver member, and a compressioncap;

FIG. 3 is a perspective, partial cut-away view of the bone anchorassembly of FIG. 2;

FIG. 4 is a cut-away view of the receiver member and the bone anchor ofthe bone anchor assembly of FIG. 2;

FIG. 5A is a partial cross-sectional view of the bone anchor assembly ofFIG. 2 before the compression cap has been attached to the receivermember;

FIG. 5B is an enlarged cross-sectional view of the compression cap andthe receiver member of the bone anchor assembly of FIG. 2 before thecompression cap has been attached to the receiver member;

FIG. 6A is a partial cross-sectional view of the bone anchor assembly ofFIG. 2 after the compression cap has been attached to the receivermember;

FIG. 6B is an enlarged cross-sectional view of the compression cap andthe receiver member of the bone anchor assembly of FIG. 2 after thecompression cap has been attached to the receiver member;

FIG. 7 is a partial perspective view of another embodiment of a boneanchor assembly having a bone anchor, a receiver member, and acompression cap, before the compression cap has been attached to thereceiver member;

FIG. 8 is a partial perspective view of the bone anchor assembly of FIG.7 after the compression cap has been attached to the receiver member;

FIG. 9 is a perspective view of the compression cap of the bone anchorassembly of FIG. 7;

FIG. 10 is a partial cross-sectional view of the bone anchor assembly ofFIG. 7; and

FIG. 11 is a partial cross-sectional perspective view of the receivermember of the bone anchor assembly of FIG. 7.

DETAILED DESCRIPTION

Certain exemplary embodiments will now be described to provide anoverall understanding of the principles of the structure, function,manufacture, and use of the devices and methods disclosed herein. One ormore examples of these embodiments are illustrated in the accompanyingdrawings. Those skilled in the art will understand that the devices andmethods specifically described herein and illustrated in theaccompanying drawings are non-limiting exemplary embodiments and thatthe scope of the present invention is defined solely by the claims. Thefeatures illustrated or described in connection with one exemplaryembodiment may be combined with the features of other embodiments. Suchmodifications and variations are intended to be included within thescope of the present invention.

Various exemplary methods and devices are provided for fixing boneanchors to bone. In general, the methods and devices can allow for abone anchor to be fixed to a bone at a desired angle relative to areceiver member. In an exemplary embodiment, a bone anchor assembly isprovided that includes a bone anchor configured to engage bone, areceiver member for seating a head of the bone anchor, and a compressionmember for securing the receiver member at fixed angle with respect tothe bone anchor. The compression member can be seated within thereceiver member, proximally of the head of the bone anchor, and caninclude one or more engagement features to facilitate attachment to thereceiver member. Similarly, the receiver member can include one or morecomplementary engagement features that correspond to the engagementfeatures of the compression member. Mating between the correspondingengagement features of the compression member and the receiver member ina secured configuration can be sufficient to inhibit or prevent removalof the compression member from the receiver member, thus reducing a riskof accidental loss or misplacement of the compression member. Engagementof the corresponding engagement features can optionally be sufficient tosubstantially prevent longitudinal and/or rotational movement of thecompression member with respect to the receiver member, such that oncethe compression member is in the secured configuration, the compressionmember can exert a distal force on the head of the bone anchor and cansubstantially fix the bone anchor at the desired angle with respect tothe receiver member.

FIGS. 1A-1D illustrate a prior art bone anchor assembly 10 that includesa bone anchor 12, a receiver member 14 for receiving a spinal fixationelement, such as a spinal rod 22, to be coupled to the bone anchor 12,and a closure mechanism 16 to capture a spinal fixation element withinthe receiver member 14 and fix the spinal fixation element with respectto the receiver member 14. The bone anchor 12 includes a proximal head18 and a distal shaft 20 configured to engage bone. The receiver member14 has a proximal end 26 having a pair of spaced apart arms 28A, 28Bdefining a recess 30 therebetween and a distal end 32 having a distalend surface 34 defining an opening through which at least a portion ofthe bone anchor 12 extends. The closure mechanism 16 can be positionablebetween and can engage the arms 28A, 28B to capture a spinal fixationelement, e.g., a spinal rod 22, within the receiver member 14 and fixthe spinal fixation element with respect to the receiver member 14.

The proximal head 18 of the bone anchor 12 is generally in the shape ofa truncated sphere having a planar proximal surface 36 and anapproximately spherically-shaped distal surface 38. The illustrated boneanchor assembly is a polyaxial bone screw designed for posteriorimplantation in the pedicle or lateral mass of a vertebra. The proximalhead 18 of the bone anchor 12 engages the distal end 32 of the receivermember 14 in a ball and socket like arrangement in which the proximalhead 18 and the distal shaft 20 can pivot relative to the receivermember 14. The distal surface 38 of the proximal head 18 of the boneanchor 12 and a mating surface within the distal end 32 of the receivermember 14 can have any shape that facilitates this arrangement,including, for example, spherical (as illustrated), toroidal, conical,frustoconical, and any combinations of these shapes.

The distal shaft 20 of the bone anchor 12 can be configured to engagebone and, in the illustrated embodiment, includes an external boneengaging thread 40. The thread form for the distal shaft 20, includingthe number of threads, the pitch, the major and minor diameters, and thethread shape, can be selected to facilitate connection with bone.Exemplary thread forms are disclosed in U.S. Patent ApplicationPublication No. 2011/0288599, filed on May 18, 2011, and in U.S.Publication No. 2013/0053901, filed Aug. 25, 2011, both of which areincorporated herein by reference. The distal shaft 20 can also includeother structures for engaging bone, including a hook. The distal shaft20 of the bone anchor 12 can be cannulated, having a central passage orcannula extending the length of the bone anchor to facilitate deliveryof the bone anchor over a guide wire in, for example, minimally-invasiveprocedures. Other components of the bone anchor assembly 10, including,for example, the closure mechanism 16, the receiver member 14, and thecompression member 60 (discussed below) can be cannulated or otherwisehave an opening to permit delivery over a guide wire. The distal shaft20 can also include one or more sidewall openings or fenestrations thatcommunicate with the cannula to permit bone in-growth or to permit thedispensing of bone cement or other materials through the bone anchor 12.The sidewall openings can extend radially from the cannula through thesidewall of the distal shaft 20. Exemplary systems for delivering bonecement to the bone anchor assembly 10 and alternative bone anchorconfigurations for facilitating cement delivery are described in U.S.Patent Application Publication No. 2010/0114174, filed on Oct. 29, 2009,which is hereby incorporated herein by reference. The distal shaft 20 ofthe bone anchor 12 can also be coated with materials to permit bonegrowth, such as, for example, hydroxyl apatite, and the bone anchorassembly 10 can be coated partially or entirely with anti-infectivematerials, such as, for example, tryclosan.

The proximal end 26 of the receiver member 14 includes a pair of spacedapart arms 28A, 28B defining a U-shaped recess 30 therebetween forreceiving a spinal fixation element, e.g., a spinal rod 22. Each of thearms 28A, 28B can extend from the distal end 32 of the receiver member14 to a free end. The arms 28A, 28B can include a feature, such as abore, recess, dimple, notch, projection, or the like, to facilitateconnection of the receiver member 14 to instruments. In the exemplaryembodiment, opposing bores 80A, 80B are formed on the arms 28A, 28B forinsertion of two pins as part of a swaging process, described in moredetail below. The outer surface of each arm 28A, 28B can further includean arcuate groove at the respective free end of the arms. Such groovesare described in more detail in U.S. Pat. No. 7,179,261, issued on Feb.20, 2007, which is hereby incorporated herein by reference. At least aportion of the proximal end surface 48 of the receiver member 14 definesa plane Y. The receiver member 14 has a central longitudinal axis L.

The distal end 32 of the receiver member 14 includes a distal endsurface 34 which is generally annular in shape defining a circularopening through which at least a portion of the bone anchor 12 extends.For example, the distal shaft 20 of the bone anchor 12 can extendthrough the opening. At least a portion of the distal end surface 34defines a plane X.

The bone anchor 12 can be selectively fixed relative to the receivermember 14. Prior to fixation, the bone anchor 12 is movable relative tothe receiver member 14 within a cone of angulation generally defined bythe geometry of the distal end 32 of the receiver member and theproximal head 18 of the bone anchor 12. The illustrated bone anchor is afavored-angle polyaxial screw in which the cone of angulation is biasedin one direction. In this manner, the bone anchor 12 is movable relativeto the receiver member 14 in at least a first direction, indicated byarrow A in FIG. 1D, at a first angle C relative to the centrallongitudinal axis L of the receiver member 14. The bone anchor 12 isalso movable in at least a second direction, indicated by arrow B inFIG. 1D, at a second angle D relative to the longitudinal axis L. Thefirst angle C is greater than the second angle D and, thus, the shaft 20of the bone anchor 12 is movable more in the direction indicated byarrow A than in the direction indicated by arrow B. The distal shaft 20of the bone anchor 12 defines a neutral axis 48 with respect to thereceiver member 14. The neutral axis 48 can be perpendicular to theplane X defined by the distal end surface 34 and intersects the centerpoint of the opening in the distal end surface 34 through which thedistal shaft 20 of the bone anchor 12 extends. The neutral axis 48 canbe oriented at an angle to the central longitudinal axis L of thereceiver member 14. The plane Y defined by at least a portion of theproximal end surface 48 of the receiver member 14 intersects the plane Xdefined by at least a portion of the distal end surface 34 of thereceiver member 12. The proximal end 26 of the receiver member 14 caninclude a proximal first bore 50 coaxial with a first centrallongitudinal axis N (which is coincident with longitudinal axis L) and adistal second bore 52 coaxial with a second central longitudinal axis M(which is coincident with the neutral axis 48) and the first centrallongitudinal axis N and second central longitudinal axis M can intersectone another. The angle between the plane X and the plane Y and the anglebetween the axis L and the axis M can be selected to provide the desireddegree of biased angulation. Examples of favored angled polyaxial screwsare described in more detail in U.S. Pat. No. 6,974,460, issued on Dec.13, 2005, and in U.S. Pat. No. 6,736,820, issued on May 18, 2004, bothof which are hereby incorporated herein by reference. Alternatively, thebone anchor assembly can be a conventional (non-biased) polyaxial screwin which the bone anchor pivots in the same amount in every directionand has a neutral axis that is coincident with the central longitudinalaxis L of the receiver member.

The spinal fixation element, e.g., the spinal rod 22, can eitherdirectly contact the proximal head 18 of the bone anchor 12 or cancontact an intermediate element, e.g., a compression member 60. Thecompression member 60 can be positioned within the receiver member 14and interposed between the spinal rod 22 and the proximal head 18 of thebone anchor 12 to compress the distal outer surface 38 of the proximalhead 18 into direct, fixed engagement with the distal inner surface ofthe receiver member 14. The compression member 60 can include a pair ofspaced apart arms 62A and 62B defining a U-shaped seat 64 for receivingthe spinal rod 22 and a distal surface 66 for engaging the proximal head18 of the bone anchor 12. A largest diameter of the compression member60 can be smaller than a smallest inner diameter of the receiver member14 to allow the compression member 60 to fit within the recess 30 of thereceiver member 14.

As best seen in FIG. 1B, the compression member 60 is configured toslide freely along the longitudinal axis L within the slot 30 of thereceiver member 14. To secure the compression cap 60 in place within thereceiver member 14, the compression member 60 is configured to bemechanically deformed against the receiver member 14. Opposing bores68A, 68B formed on the arms 62A, 62B of the compression member 60 arealigned with the bores 80A, 80B of the receiver member 14, such thatopposing pins can be inserted through the passageways defined by bores68A, 80A and bores 68B, 80B to compress or “swage” the compressionmember 60 against the receiver member 14. The swaging process canprevent subsequent removal of the compression member 60 from thereceiver member 14.

The proximal end 26 of the receiver member 14 can be configured toreceive a closure mechanism 16 positionable between and engaging thearms 28A, 28B of the receiver member 14. The closure mechanism 16 can beconfigured to capture a spinal fixation element, e.g., a spinal rod 22,within the receiver member 14, to fix the spinal rod 22 relative to thereceiver member 14, and to fix the bone anchor 12 relative to thereceiver member 14. The closure mechanism 16 can be a single set screwhaving an outer thread for engaging an inner thread 42 provided on thearms 28A, 28B of the receiver member 14. In the illustrated embodiment,however, the closure mechanism 16 comprises an outer set screw 70positionable between and engaging the arms 28A, 28B of the receivermember 14 and an inner set screw 72 positionable within the outer setscrew 70. The outer set screw 70 is operable to act on the compressionmember 60 to fix the bone anchor 12 relative to the receiver member 14.The inner set screw 72 is operable to act on the spinal rod 22 to fixthe spinal rod 22 relative to the receiver member 14. In this manner,the closure mechanism 16 permits the bone anchor 12 to be fixed relativeto the receiver member 14 independently of the spinal rod 22 being fixedto the receiver member 14. In particular, the outer set screw 70 canengage the proximal end surfaces of the arms 62A, 62B of the compressionmember 60 to force the distal surface 66 of the compression member 60into contact with the proximal head 18 of bone anchor 12, which in turnforces the distal surface 38 of the proximal head 18 into fixedengagement with the distal inner surface of the receiver member 14. Theinner set screw 72 can engage the spinal rod 22 to force the spinal rod22 into fixed engagement with the rod seat 64 of the compression member60.

The outer set screw 70 includes a first outer thread 74 for engaging acomplementary inner thread 42 on the arms 28A, 28B of the receivermember 14. The outer set screw 74 includes a central passage 96 from atop surface 98 of the outer set screw 74 to a bottom surface 100 of theouter set screw 74 for receiving the inner set screw 72. The centralpassage 96 can includes an inner thread 102 for engaging a complementaryouter thread 104 on the inner set screw 72. The thread form for theinner thread 102 and the outer thread 104, including the number ofthreads, the pitch, major and minor diameter, and thread shape, can beselected to facilitate connection between the components and transfer ofthe desired axial tightening force. The top surface 98 of the outer setscrew 74 can have one or more drive features to facilitate rotation andadvancement of the outer set screw 74 relative to the receiver member14. The illustrated outer set screw 74 includes drive features in theform of a plurality of cut-outs 106 spaced-apart about the perimeter ofthe top surface 98. The inner set screw 72 can include drive featuresfor receiving an instrument to rotate and advance the inner set screw 72relative to the outer set screw 74. The illustrated inner set screw 72includes drive features in the form of a central passage 108 having aplurality of spaced apart, longitudinally oriented cut-outs for engagingcomplementary features on an instrument.

The bone anchor assembly 10 can be used with a spinal fixation elementsuch as rigid spinal rod 22. The various components of the bone anchorassemblies disclosed herein, as well as the spinal rod 22, can beconstructed from various materials, including titanium, titanium alloys,stainless steel, cobalt chrome, PEEK, or other materials suitable forrigid fixation. In other embodiments, the spinal fixation element can bea dynamic stabilization member that allows controlled mobility betweenthe instrumented vertebrae.

In use, bone can be prepared to receive the bone anchor assembly 10,generally by drilling a hole in the bone which is sized appropriately toreceive the bone anchor 12. If not already completed, the bone anchorassembly 10 can be assembled, which can include assembling the boneanchor 12 and the receiver member 14, so that the distal shaft 20extends through the opening in the distal end 32 of the receiver member14 and the proximal head 18 of the bone anchor 12 is received in thedistal end 32 of the receiver member 14. A driver tool can be fittedwith the bone anchor 12 to drive the bone anchor 12 into the preparedhole in the bone. The compression member 60 can be positioned within thereceiver member 14 such that the arms 62A, 62B of the compression memberare aligned with the arms 28A, 28B of the receiver member 14 and thedistal surface 66 of the compression member 60 is in contact with theproximal head 18 of the bone anchor 12. A spinal fixation element, e.g.,the spinal rod 22, can be located in the recess 30 of the receivermember 14. The closure mechanism 16 can be engaged with the inner thread42 provided on the arms 28A, 28B of the receiver member 14.

One or more embodiments of inventive bone anchor assemblies aredescribed below. Except as indicated below, the structure, operation,and use of these embodiments is similar or identical to that of the boneanchor assembly 10 described above. Accordingly, a detailed descriptionof said structure, operation, and use is omitted here for the sake ofbrevity.

FIGS. 2-6 illustrate an exemplary embodiment of a bone anchor assembly110 that includes a compression cap 160 configured to mate to and engagea receiver member 114 such that the compression cap 160 is retainedwithin the receiver member 114. The compression cap 160 is configured tobe seated within a recess 130 of the receiver member 114, proximally ofthe bone anchor 112, and the compression cap 160 can include featuresformed thereon that engage corresponding engagement features formed onthe receiver member 114. Once mated to the receiver member 114, thecompression cap 160 can apply a frictional force to the bone anchor 112to substantially maintain the anchor 112 at a desired angle with respectto the receiver member 114, while still allowing the angular orientationof the anchor 112 to be adjusted. Engagement of the correspondingengagement features can be sufficient to prevent removal of thecompression cap 160 from the receiver member 114, and optionally tosubstantially prevent longitudinal and/or rotational movement of thecompression cap 160 with respect to the receiver member 114 when noforce is applied thereto.

Engagement features of the compression cap 160 for engaging the receivermember 114 can be formed anywhere on a surface of the compression cap160 that is configured to contact the receiver member 114. In theillustrated embodiment, the engagement features are in the form of wings161A, 161B that project radially outward from outward-facing surfaces ofthe opposed arms 162A, 162B, distally of flanges 165A, 165B on the arms162A, 162B. The wings 161A, 161B can extend around an entire width ofeach of the arms 162A, 162B measured along a circumference of thecompression cap 160, although it will be appreciated that the widthW_(C) of the wings 161A, 161B can be smaller than the width of the arms162A, 162B, and can be either the same or different from one another.Moreover, it will be appreciated that although the illustrated wings161A, 161B extend in a plane that is substantially perpendicular to thelongitudinal axis L₁ of the compression cap 160, the wings 161A, 161Bcan extend in any plane, either the same or different from one another.

The longitudinal position of the wings 161A, 161B along the longitudinallength of the compression cap 160 can vary, but preferably the wings161A, 161B are formed at a location that corresponds to a location ofcomplementary engagement features in the receiver member 114, discussedbelow, and at a location that will retain the compression cap 160 withinthe receiver member 114 at a predetermined position just proximal to thehead of the bone anchor 112, and more preferably at a predeterminedposition effective to apply some amount of frictional force to the headof the bone anchor 112. As shown in FIG. 2, the wings 161A, 161B arelocated distal to the flanges 165A, 165B and proximal to the distal end166 and also just proximal to the distal-most end of the U-shapedcut-out 164 formed in the compression cap 160.

The wings 161A, 161B can have any shape and dimension, either the sameor different from one another. In the illustrated embodiment, the wings161A, 161B are in the form of elongate protrusions or ridges formed onthe outer surface of each arm 162A, 162B. The wings 161A, 161B have awidth W_(C) measured horizontally about a circumference of thecompression cap 160, a height H_(C) measured along a longitudinal axisL₁ of the compression cap 160, and a depth or thickness T_(C) measuredalong a radial axis of the compression cap 160. The width W_(C) of thewings 161A, 161B can vary depending on the total width of the arms 162A,162B, and the width W_(C) can be equal to or less than a total width ofthe arms 162A, 162B. The height H_(C) and thickness T_(C) of the wings161A, 161B can also vary, but preferably the height H_(C) and thicknessT_(C) are small enough to allow for insertion of the compression cap 160into the recess 130 of the receiver member 114. In some embodiments, thewings 161A, 161B have a height H_(C) and thickness T_(C) that are smallenough to allow the wings 161A, 161B to deflect as the compression cap160 is inserted into the receiver member 114, as described in moredetail below. Although the engagement features of the illustratedembodiment are in the form of wings 161A, 161B, it will be appreciatedby a person skilled in the art that the compression cap 160 can have avariety of engagement features thereon for engaging the receiver member114. Moreover, although there are only two wings 161A, 161B in theillustrated embodiment, it will be appreciated by a person skilled inthe art that the compression cap 160 can have any number of engagementfeatures formed thereon.

One or more complementary engagement features can be formed on thereceiver member 114. In the illustrated embodiment, best shown in FIG.4, the receiver member 114 includes pockets 124A, 124B formed on innerwalls of the opposed arms 128A, 128B. The pockets 124A, 124B areconfigured to seat the wings 161A, 161B of the compression cap 160 whenthe compression cap 160 is in a secured configuration. The location ofthe pockets 124A, 124B within the receiver member 114 can vary, but asindicated above with respect to the compression cap 160, the pockets124A, 124B are preferably at a location that seats the compression cap160 in a predetermined position that is proximal of the head of the boneanchor 112. The location can also be configured to position the u-shapedcut-outs formed between the arms 162A, 162B of the compression cap 160in alignment with u-shaped cut-outs formed between the arms 128A, 128Bof the receiver member 114. In the illustrated embodiment, the pockets124A, 124B are located distal of the threads 142 formed in a proximalportion of the receiver member 114, and proximal of the distal-most endof the u-shaped cut-outs formed between the arms 128A, 128B.

The pockets 124A, 124B can have any shape and dimension, either the sameor different from one another, but they preferably have a shape and sizethat allows the wings 161A, 161B to be seated and retained therein. Atleast one dimension of the pockets 124A, 124B can be substantially thesame as or slightly smaller than a corresponding dimension of the wings161A, 161B to provide for an interference fit between the wings 161A,161B and the pockets 124A, 124B, and/or at least one dimension of thepockets 124A, 124B can be slightly larger than a corresponding dimensionof the wings 161A, 161B to allow for some adjustment of the compressioncap 160 within the recess 130 when the compression cap 160 is in thesecured configuration. In an exemplary embodiment, the pockets 124A,124B have a width W_(R) measured horizontally about a circumference ofthe receiver member 114 that is substantially the same as the widthW_(C) of the wings 161A, 161B, a depth D_(R) measured along a radialaxis of the receiver member 114 that is substantially the same as thethickness T_(C) of the wings 161A, 161B, and a height L_(R) measuredalong the longitudinal axis L₁ of the compression cap 160 that issubstantially the same as a height H_(C) of the wings 161A, 161B. Atotal diameter between an outer-most surface of each of the wings 161A,161B can also be greater than an inner diameter of the receiver member114 so as to prevent removal of the compression cap 160 once mated withthe receiver member 114. A tight fit between at least two of thecorresponding dimensions, e.g., between the width W_(C) of thecompression cap 160 and the width W_(R) of the receiver member 114,and/or between the height H_(C) of the compression cap 160 and theheight H_(R) of the receiver member 114, and/or between the thicknessT_(C) of the compression cap 160 and the depth D_(R) of the receivermember 114, can help to substantially prevent both longitudinal androtational movement of the compression cap 160 once the compression cap160 is in the secured configuration. A small difference between theheight H_(C) of the wings 161A, 161B and the height H_(R) of the pockets124A, 124B can allow for proximal-distal movement of the compression cap160, thus allowing for further tightening of the compression cap 160against the bone anchor 112. This can be achieved by applying a closuremember (not shown) to the receiver member 114 so as to cause a spinalfixation rod seated within the receiver member 114 to apply a distalforce to the compression cap 160 to thereby cause the compression cap160 to frictionally engage and lock the bone anchor 112 in a fixedposition relative to the receiver member 114. In addition, as will beexplained in more detail below, the additional space within the pockets124A, 124B along the longitudinal axis L₁ of the compression cap 160 canallow for the bone anchor 112 to assume various angles with respect tothe longitudinal axis L₁ of the compression cap 160 prior to locking thebone anchor assembly. In an exemplary embodiment, the relative heightsH_(C), H_(R) are configured so as to cause the compression cap 160 toapply friction to the bone anchor 112 to maintain the bone anchor 112 ata desired angle when no closure mechanism is applied to the receivermember 114. The friction can be overcome by applying a force to thereceiver member 114 to adjust the angle as desired.

As indicated above, the corresponding engagement features of thecompression cap 160 and the receiver member 114 can be configured tofacilitate insertion of the compression cap 160 into the receiver member114, but to inhibit or prevent removal of the compression cap 160 fromthe receiver member 114. As the compression cap 160 is distally advancedfrom an unsecured configuration, shown in FIG. 5, where the wings 161A,161B are proximal of and not seated within the pockets 124A, 124B,toward a secured configuration, shown in FIG. 6, where the wings 161A,161B are seated within the pockets 124A, 124B, a user can apply adistally-directed force to overcome the resistive force of the wings161A, 161B against the inner walls of the receiver member 114. Theresistive force can be reduced by complementary lead-in surfacegeometries of the wings 161A, 161B and the receiver member 114. In theillustrated embodiment, the wings 161A, 161B have ramped distal-facingsurfaces that are complementary to ramped shoulders 125A, 125B of thearms 128A, 128B of the receiver member 114, such that the wings 161A,161B can slide along the shoulders 125A, 125B of the arms 128A, 128B asthe compression cap 160 is advanced distally into the recess 130.

To further facilitate insertion of the compression cap 160 into thereceiver member 114, one or all of the wings 161A, 161B, the cap's arms162A, 162B, and the receiver's arms 128A, 128B can be formed from one ormore flexible materials. In one embodiment, the wings 161A, 161B can beformed from one or more flexible materials that can allow the wings161A, 161B to deflect or bend proximally as the wings 161A, 161B passthrough the receiver member 114 and into the pockets 124A, 124B. Inanother embodiment, the wings 161A, 161B can be formed from a shapememory material that can allow the wings 161A, 161B to compress ordeform inwardly as the wings 161A, 161B pass through the receiver member114 and then expand once the wings 161A, 161B are seated within thepockets 124A, 124B. In another embodiment (not shown), the wings 161A,161B can be biased inwardly, e.g., by springs, and can deflect outwardlyas the compression cap 160 is advanced through the receiver member 114and then return to the biased inward position once the wrings 161A, 161Bare aligned with the pockets 124A, 124B. As the wings 161A, 161B passinto the pockets 124A, 124B, the wings 161A, 161B can snap intoengagement with the pockets 124A, 124B, thus providing tactile feedbackto a user and helping to inhibit or prevent subsequent removal of thecompression cap 160 from the receiver member 114. Similarly, the arms162A, 162B of the compression cap 160 can deflect inwardly as thecompression cap 160 is inserted into the recess 130, and/or the arms128A, 128B of the receiver member 114 can deflect outwardly as thecompression cap 160 is inserted into the recess 130.

To inhibit or prevent removal of the compression cap 160 from thereceiver member 114, the wings 161A, 161B can have proximal-facingsurfaces that extend in a plane substantially perpendicular to thelongitudinal axis L₁ of the compression cap 160. Once the wings 161A,161B are seated within the pockets 124A, 124B in the securedconfiguration, the proximal-facing surfaces of the wings 161A, 161B canabut distal-facing surfaces of the pockets 124A, 124B, which can alsoextend in a plane substantially perpendicular to the longitudinal axisL₁ of the compression cap 160. In addition, the wings 161A, 161B can beconfigured such that distal or outward flexion of the wings 161A, 161Baway from the compression cap 160 is more difficult to achieve than anyproximal or inward flexion of the wings 161A, 161B toward thecompression cap 160 that is required for insertion of the wings 161A,161B into the pockets 124A, 124B, e.g., the wings 161A, 161B can bebiased proximally.

In one embodiment, once the compression cap 160 is seated within thereceiver member 114 in the secured configuration, with the wings 161A,161B extending into the pockets 124A, 124B, a distal-facing surface 166of the compression cap 160 can exert a frictional force on a proximalhead 118 of the bone anchor 112 to substantially maintain the head 118in a fixed position relative to the receiver member 114. The forceshould be sufficient to prevent free movement of the bone anchor 112relative to the receiver member 114 while still allowing a user to movethe receiver member 114 relative to the bone anchor 112. The force canvary slightly when the compression cap 160 is in the securedconfiguration, as the compression cap 160 can be capable of somelongitudinal movement. As explained above, the pockets 124A, 124B canhave a slightly greater height H_(R) than a corresponding height H_(C)of the wings 161A, 161B, thereby allowing for slight movement of thewings 161A, 161B and thus the compression cap 160 along a longitudinalaxis L₁ of the compression cap 160. This can be necessary to allow forminor adjustments to the angle of the bone anchor 112 and for furthertightening of the compression cap 160 against the proximal head 118. Asshown in FIGS. 5A and 6A, the proximal head 118 of the bone anchor 112can be substantially spherical to match a corresponding spherical distalsurface 166 of the compression cap 160, thus allowing for articulationof the bone anchor 112 at an angle to the longitudinal axis L₁ of thecap 160. When the compression cap 160 is distally advanced and in alocked configuration, e.g., by applying a closure mechanism (not shown)to the receiver member 114, the spherical distal surface 166 of thecompression cap 160 can engage the proximal head 118 of the bone anchor112 to thereby lock the bone anchor at a fixed position relative to thereceiver member 114. While not shown, in certain embodiments the distalspherical surface 166 of the compression cap 160 and the proximal head118 of the bone anchor 112 can each include opposed flats formed thereonand configured to limit angular movement of the bone anchor 112 to asingle plane of motion, e.g., to thereby provide uniplanar motion.

The corresponding engagement features of a compression member and areceiver member can be of any shape and size, and can be configured toengage each other in a variety of ways. For example, an additionalembodiment of a bone anchor assembly 210 having corresponding engagementfeatures formed on a compression member and a receiver member isillustrated in FIGS. 7-11. The bone anchor assembly 210, as well asother bone anchor assemblies described herein, can generally beconfigured and used similar to the bone anchor assembly 110 of FIGS.2-6. Additionally, like-named elements and like-illustrated elements ofthe bone anchor assembly 110 and of the other bone anchor assembliesdiscussed herein can be configured and used similar to one another.

FIGS. 7-11 illustrate the bone anchor assembly 210 having a compressioncap 260 configured to secure a bone anchor 212 at a fixed angle to areceiver member 214. Corresponding engagement features formed on thecompression cap 260 and the receiver member 214 can be configured toengage one another upon rotation of the compression cap 260 with respectto the receiver member 214. Thus, rotation of the compression cap 260can move the compression cap 260 from an unsecured configuration, shownin FIG. 7, where the corresponding engagement features are unengaged, toa secured configuration, shown in FIG. 8, where the correspondingengagement features are engaged. In particular, a user can distallyadvance the compression cap 260 into a recess 230 of the receiver member214 such that opposed arms 262A, 262B of the compression cap 260 areoffset from opposed arms 228A, 228B of the receiver member 214, and thenrotate the compression cap 260 while applying a distal force to causethe opposed arms 262A, 262B of the compression cap 260 to be alignedwith the opposed arms 228A, 228B. In the illustrated embodiment, theangle of rotation required to move the compression cap 260 between theunsecured and the secured configurations is approximately 90 degrees,although the angle of rotation can vary from about 10 to 90 degreesdepending on various factors, e.g., the dimensions of the opposed arms262A, 272B of the compression cap 260 relative to the opposed arms 228A,228B of the receiver member 214, the dimensions and location of thecorresponding engagement features, etc.

Engagement features of the compression cap 260 can be a variety ofshapes and sizes, although in the illustrated embodiment the engagementfeatures are in the form of wings 261A, 261B extending radially from theopposed arms 262A, 262B of the compression cap 260 and configured toengage complementary engagement features of the receiver member 214. Asbest seen in FIG. 9, both proximal and distal-facing surfaces of thewings 261A, 261B on the compression cap 260 are upwardly sloped atsubstantially the same angle to a longitudinal axis L₂ of thecompression cap 260, such that radially outermost edges of the wings261A, 261B are proximal of radially innermost edges of the wings 261A,261B. The sloped surfaces of the wings 261A, 261B can thus allow distaladvancement of the compression cap 260 into the receiver member, whilesubstantially preventing proximal movement of the compression cap 260and reducing a likelihood of loss or removal of the compression cap 260once it is in the secured configuration. The compression cap 260 can beadjusted longitudinally while in the secured configuration, however, toallow for additional tightening of the compression cap 260 against thebone anchor 212 and/or for articulation of the bone anchor 212 to adesired angle, as discussed above.

In this embodiment, shelves 242A, 242B extend radially inward from innerwalls of the arms 228A, 228B of the receiver member 214 and areconfigured to engage the wings 261A, 261B of the compression cap 260.The shelves 242A, 242B can be disposed anywhere along the inner walls ofthe arms 228A, 228B, although in the illustrated embodiment the shelves242A, 242B are disposed distally of threads 242 that can be configuredto engage a closure mechanism, e.g., a set screw. As shown in FIG. 10,cut-outs 224A, 224B formed on distal-facing sides of the shelves 242A,242B can be provided for receiving the wings 261A, 261B when thecompression cap 260 is in the secured configuration, although it will beappreciated by a person skilled in the art that the shelves 242A, 242Bdo not require any modifications to engage the wings 261A, 261B. Inanother embodiment (not shown), any one or more of the threads 242 canbe configured to engage the wings 261A, 261B.

Where provided, the cut-outs 224A, 224B of the receiver member 214 canbe configured to facilitate transition of the compression cap 260 intothe secured configuration, but to inhibit or prevent transition of thecompression cap 260 out of the secured configuration. In the illustratedembodiment, shown in FIG. 11, the cut-outs 224A, 224B can havesubstantially the same dimensions as the wings 261A, 261B, although thecut-outs 224A, 224B can have at least one dimension that is slightlysmaller than a corresponding dimension of the wings 261A, 261B to createan interference fit between the wings 261A, 261B and the cut-outs 224A,224B and/or to allow for a snapping effect as the wings 261A, 261B areinserted into the cut-outs 224A, 224B. By way of non-limiting example, awidth W_(C) of the wings 261A, 261B measured around a circumference ofthe compression cap 260 can be slightly larger than a width W_(R) of thecut-outs 224A, 224B measured around a circumference of the receivermember 214, bounded on either end by ramped edges 241. The ramped edges241 can have a slope inclined upwards towards a center of each of thecut-outs 224A, 224B, and planar side surfaces that abut opposite ends ofthe wings 261A, 261B when the compression cap 260 is in the securedposition. The ramped edges 241 can thus provide a lead-in surfacegeometry as the wings 261A, 261B are rotated into the securedconfiguration and/or can inhibit removal of the compression cap 260 fromthe receiver member 214 by rotation once the compression cap 260 is inthe secured configuration.

As with the previous embodiment, the wings 261A, 261B can be formed froma flexible material that is capable of being distally deflected as auser rotates the compression cap 260 from the unsecured to the securedconfiguration, such that the wings 261A, 261B are slightly bent by theramped edges 241 as the compression cap 260 is rotated into the securedconfiguration and/or such that the wings 261A, 261B snap into alignmentwith the cut-outs 224A, 224B. In another embodiment, not shown, thewings 261A, 261B remain distally deflected while in the securedconfiguration, thus further securing the compression cap 260 to thereceiver member 214 and providing additional distal force against thebone anchor 212.

Although the illustrated embodiments of bone anchor assemblies includemale engagement features on a compression cap and female engagementfeatures on a corresponding receiver member, the engagement features canbe reversed with the compression cap having female engagement featuresand the receiver member having corresponding male engagement features.By way of non-limiting example, the receiver member can have wingsextending from inner walls thereof that can be configured to engagerecesses formed on outer surfaces of the compression cap. The wings andcorresponding recesses can be configured and used similarly to thosedescribed above for bone anchor assemblies 110, 210.

In use, a bone anchor assembly can be assembled, either duringmanufacturing or intraoperatively, by passing an elongate shank of abone anchor in a proximal-to-distal direction through an aperture formedin a distal end of a receiver member. A proximal head portion of thebone anchor can be polyaxially seated in a spherical recess formed in adistal portion of the receiver member. A compression member can beinserted between the opposed arms of the receiver member, proximal tothe proximal head of the bone anchor. Corresponding engagement featuresof the compression member and the receiver member can be engaged byurging the compression member distally within the receiver member, or,in another aspect, by rotating and distally advancing the compressionmember into the receiver member. Engagement of the correspondingengagement features in a secured configuration can prevent proximalmovement of the compression member with respect to the receiver member,thus securing the compression member within the receiver member. Suchengagement can also cause the compression member to apply a frictionalforce to the head of the bone anchor to maintain the bone anchor at afixed angle with respect to the receiver member. An angle of the boneanchor with respect to the receiver member can be adjusted by applying aforce sufficient to overcome the frictional force.

The assembled bone anchor can be implanted in a bone of a patient. Thebone can be prepared to receive the bone anchor assembly, e.g., bydrilling an appropriately sized hole. A driver tool can be fitted withthe bone anchor to drive the bone anchor into the prepared hole in thebone. A spinal fixation element, e.g., a rod, can be located in betweenthe arms of the receiver member. A closure mechanism can be engaged withan inner thread formed on opposed arms of the receiver member,proximally of the rod, which can urge the compression member furtherdistally to exert a frictional force on the head of the bone anchor andthus lock the bone anchor at a fixed angle with respect to the receivermember.

Although the invention has been described by reference to specificembodiments, it should be understood that numerous changes may be madewithin the spirit and scope of the inventive concepts described.Accordingly, it is intended that the invention not be limited to thedescribed embodiments, but that it have the full scope defined by thelanguage of the following claims.

The invention claimed is:
 1. A bone screw assembly, comprising: a screwhaving a proximal head portion and a distal shank portion; a receivermember polyaxially coupled to the head portion of the screw; acompression cap disposed within the receiver member and having a distalend configured to engage the head portion of the screw, the compressioncap including opposed projections extending radially therefromconfigured to mate with complementary recesses formed in the receivermember, the compression cap being retained within the receiver memberwhile still allowing polyaxial movement of the receiver member relativeto the screw, wherein at least one dimension of the recesses is largerthan a corresponding dimension of the projections to allow for someadjustment of the compression cap when the projections are seated in therecesses, wherein the compression cap includes first and seconddeformable arms having free ends configured to deflect inwards duringassembly of the compression cap to the receiver member and to thendeflect radially outward when the compression cap is seated in thereceiver member, wherein the compression cap is positionable withrespect to the receiver member in a first, distal position in whichramped outer surfaces of the arms engage a distal ramped surface of theinterior of the receiver member and a second, proximal position in whichthe ramped outer surfaces of the arms do not engage and are proximal tothe distal ramped surface of the interior of the receiver member, andwherein the ramped outer surfaces of the arms of the compression capmove towards each other as the arms are deflected inwards.
 2. The bonescrew assembly of claim 1, wherein the opposed projections and thecomplementary recesses are effective, when mated, to maintain thecompression cap in a substantially fixed rotational position relative tothe receiver member.
 3. The bone screw assembly of claim 1, wherein aproximal-facing surface of each projection is planar and wherein adistal-facing surface of each recess is planar.
 4. The bone screwassembly of claim 1, wherein an outside diameter of the compression capis less than an inside diameter of the receiver member.
 5. The bonescrew assembly of claim 1, wherein a distance between outer surfaces ofthe opposed projections is greater than an inside diameter of thereceiver member.
 6. The bone screw assembly of claim 1, wherein rotatingthe compression cap relative to the receiver member is effective to movethe opposed projections into engagement with the complementary recesses.7. The bone screw assembly of claim 1, wherein the complementaryrecesses comprise cut-outs formed in a shelf that extends radiallyinward from an interior sidewall of the receiver member.
 8. The bonescrew assembly of claim 1, wherein a width of each projection measuredhorizontally about the circumference of the compression cap is greaterthan a thickness of the opposed projection measured along a radial axisof the compression cap.
 9. The bone screw assembly of claim 1, whereinthe recesses formed in the receiver member have a depth measured along aradial axis of the receiver member that is substantially the same as athickness of the opposed projections.
 10. The bone screw assembly ofclaim 1, wherein the ramped outer surfaces of the arms are spaced apartfrom the opposed projections along a longitudinal axis of thecompression cap.
 11. A bone screw assembly, comprising: a screw having aproximal head portion and a distal shank portion; a receiver memberconfigured to receive the head portion of the screw therein; acompression cap disposed within the receiver member and includingopposed projections extending radially outward from the compression capand configured to mate with recesses formed in the receiver member tolimit rotational movement of the compression cap with respect to thereceiver member, the compression cap further including one or more armshaving free ends configured to deflect inwards during assembly of thecompression cap to the receiver member and to then deflect radiallyoutward into a cavity formed in the receiver member; wherein the headportion of the screw is polyaxially movable with respect to thecompression cap and the receiver member; wherein the compression cap ispositionable with respect to the receiver member in a first, distalposition in which ramped outer surfaces of the one or more arms engage adistal ramped surface of the interior of the receiver member and asecond, proximal position in which the ramped outer surfaces of the armsdo not engage and are proximal to the distal ramped surface of theinterior of the receiver member; wherein the ramped outer surfaces ofthe one or more arms move towards each other as the arms are deflectedinwards.
 12. The bone screw assembly of claim 11, wherein the opposedprojections and the complementary recesses are effective, when mated, tomaintain the compression cap in a substantially fixed rotationalposition relative to the receiver member.
 13. The bone screw assembly ofclaim 11, wherein a proximal-facing surface of each projection is planarand wherein a distal-facing surface of each recess is planar.
 14. Thebone screw assembly of claim 11, wherein an outside diameter of thecompression cap is less than an inside diameter of the receiver member.15. The bone screw assembly of claim 11, wherein a distance betweenouter surfaces of the opposed projections is greater than an insidediameter of the receiver member.
 16. The bone screw assembly of claim11, wherein rotating the compression cap relative to the receiver memberis effective to move the opposed projections into engagement with thecomplementary recesses.
 17. The bone screw assembly of claim 11, whereinthe complementary recesses comprise cut-outs formed in a shelf thatextends radially inward from an interior sidewall of the receivermember.
 18. The bone screw assembly of claim 11, wherein a width of eachprojection measured horizontally about the circumference of thecompression cap is greater than a thickness of the opposed projectionmeasured along a radial axis of the compression cap.
 19. The bone screwassembly of claim 11, wherein the recesses formed in the receiver memberdo not extend entirely through the receiver member.